All braking will transfer the weight from the rear wheel(s) to the front wheel(s), regardless which brake you're using. So braking with only the rear brake will

All braking will transfer the weight from the rear wheel(s) to the front wheel(s), regardless which brake you're using. So braking with only the rear brake will transfer wight to the front wheels until you loose traction on the rear wheel and just skids. But this shift in weight can be used to initiate braking: a touch of rear brake will shift the weight forward so that your front wheel gets more traction, and then you apply the front brake. This is even more apparent on a motorcycle.